1. Technical Field
The present disclosure relates to a biasing circuit for an acoustic transducer, in particular a MEMS (Micro-Electro-Mechanical Systems) capacitive microphone, to which the following treatment will make explicit reference, without this implying any loss of generality.
2. Description of the Related Art
As is known, an acoustic transducer of a capacitive type, for example a MEMS microphone, generally comprises a microelectromechanical sensing structure including a mobile electrode, provided as a diaphragm or a membrane, set facing a fixed electrode, to provide the plates of a variable-capacitance sensing capacitor. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whereas a central portion thereof is free to move or bend in response to the pressure exerted by incident sound waves. The mobile electrode and the fixed electrode provide a capacitor, and bending upwards or downwards of the membrane that constitutes the mobile electrode causes a variation of capacitance of this capacitor. In use, the capacitance variation, which is a function of the acoustic signal to be detected, is converted into an electrical signal, which is supplied as output signal of the acoustic transducer.
In greater detail, and with reference to FIG. 1, a sensing structure 1 of a MEMS capacitive microphone, of a known type, comprises a substrate 2 of semiconductor material, for example silicon; a cavity 3 (generally known as “back chamber”) is formed in the substrate 2, for example via chemical etching from the back. A membrane, or diaphragm, 4 is coupled to the substrate 2 and closes the cavity 3 at the top. The membrane 4 is flexible and, in use, undergoes deformation as a function of the pressure of the incident sound waves coming from the cavity 3. A rigid plate 5 (generally known as “backplate”) is set above the membrane 4 and facing it via interposition of spacers 6 (for example, of insulating material, such as silicon oxide) for defining an empty space (the so-called “air gap”). The rigid plate 5 constitutes the fixed electrode of a variable-capacitance capacitor, the mobile electrode of which is constituted by the membrane 4, and has a plurality of holes 7, for example with circular cross-section, which are designed to enable free circulation of air towards the membrane 4.
MEMS capacitive microphones require an appropriate electrical biasing so that they may be used as transducers of acoustic signals into electrical signals. In general, MEMS capacitive microphones operate in the charge-biasing condition.
In order to guarantee sufficient performance for common applications, these microphones are biased at a high D.C. voltages (for example, 15 to 20 V), typically much higher than the supply voltages at which a corresponding read circuit is supplied (logic voltages, for example of 1.6 to 3 V).
For this purpose, it is common to use voltage-booster circuits, in particular of the charge-pump type made using integrated technology, which are able to generate high voltages starting from reference voltages. In general, it is known that, the higher the biasing voltage of the microphone, the greater the resulting sensitivity of the same microphone in detecting acoustic signals.
A biasing circuit 8 that has been proposed (illustrated in FIG. 2) thus envisages a charge-pump circuit, shown schematically and designated as a whole by 9, having an output terminal 9a, on which a boosted voltage, or pump voltage, VCP, is Present, that is generated starting from a supply voltage of a lower value.
The output terminal 9a is connected to a first terminal (constituted, for example, by the backplate 5) of the sensing structure 1 of the MEMS microphone (represented schematically with the equivalent circuit of a variable-capacitance capacitor CMEMS), with interposition of an insulating circuit element, with very high impedance (for example, typically with a value in the region of tera-ohms), designated by 10 and represented schematically as a resistor having resistance RB.
A second terminal (for example, constituted by the membrane 4) of the sensing structure 1 is instead connected to a reference potential of the circuit, for example ground.
The aforesaid first terminal consequently constitutes a first high-impedance node N1 associated to the insulating circuit element 10, and is further connected to a read stage 11, illustrated schematically, which receives the voltage, designated by VMEMS, present on the same first terminal, and generates an output voltage Vout, which is indicative of the detected acoustic signal.
The read stage 11 is usually provided in an integrated manner as an ASIC (Application Specific Integrated Circuit), in a die of semiconductor material, distinct with respect to the die in which the sensing structure 1 of the MEMS microphone is provided. The two dice may further be housed in the same package, or else in distinct packages, electrically connected together.
The biasing circuit 8 may also be integrated in the die in which the read circuit 11 is provided, or else be provided in a distinct die, which is housed in a same package.
The insulating circuit element 10 has insulation functions for the MEMS microphone, insulating the charge stored in the capacitor of the MEMS microphone starting from frequencies higher than a few hertz (in other words, the resulting cutoff frequency is well below the audio band, comprised between 20 Hz and 20 kHz). Given that, for frequencies in the audio band, the charge stored in the capacitor is fixed, an acoustic signal incident upon the membrane of the sensing structure 1 modulates the air gap and thus the voltage VMEMS.
The presence of the insulating circuit element 10 further appropriately attenuates both the ripple and the noise at output from the charge-pump circuit 9, forming a filtering module with the capacitance of the MEMS microphone.
Given that, in a known way, it is not possible in integrated-circuit technology to provide resistors with such high values of resistance, use of nonlinear devices has been proposed which are able to provide the high resistance values for the insulating circuit element 10.
For instance, it has been proposed for this purpose to use at least one pair of diode elements in antiparallel configuration, which provide a sufficiently high resistance, when a voltage drop of a low value (depending upon the technology, for example in the region of 100 mV) is present thereon, so as not to cause them to turn on. The same diode elements may further be obtained with transistors, appropriately diode-connected.
The biasing circuit 8 further includes a switch element 12, connected in parallel to the insulating circuit element 1. The function of this switch element 12 is to overcome the problem represented by a long start-up time of the biasing circuit 8 when it is turned on, or when it returns from a so-called “stand-by” or “power-down” condition (during which the device itself is partially turned off to go into an energy-saving condition), i.e., when it is again electrically supplied.
The insulating circuit element 10, on account of the high impedance, in fact determines with the capacitance of the MEMS microphone a high time constant.
The switch element 12 may thus be selectively operated, as a function of a control signal VSW, to provide a direct low-impedance connection between the first terminal of the sensing structure 1 and the output terminal 9a of the charge-pump circuit 9 (on which the pump voltage VCP is present), during the aforesaid start-up step.
In particular, the switch element 12 receives the control signal VSW from a control logic (not illustrated herein) so that it may be closed during the phase of start-up of the biasing circuit 8, and thus guarantee a fast settling of the first terminal of the sensing structure 1 to the desired biasing values, and to be open during a subsequent phase of normal operation of the biasing circuit 8, thus guaranteeing both proper biasing of the first terminal and insulation and noise performance guaranteed through the insulating circuit element 10.
The start-up phase terminates after the capacitor of the MEMS microphone is charged at the desired biasing voltage, i.e., at the pump voltage VCP.
In other words, the switch element 12 thus enables bypassing of the insulating circuit element 10 for a certain interval of time subsequent to supply of the biasing circuit 8, and then opens and re-establishes the connection between the sensing structure 1 of the MEMS microphone and the insulating circuit element 10, when the capacitance of the MEMS microphone has reached a sufficient value of charge and the output voltage VMEMS has a desired D.C. biasing value.
The present Applicant has, however, realized that the biasing circuit 8 described previously has at least one drawback that does not enable full exploitation of its advantages.
This drawback is linked to the presence of parasitic currents (commonly defined as “leakage currents”), at the terminal in common between the sensing structure 1 of the MEMS microphone and the insulating circuit element 10, in the example at the first high-impedance node N1 (coinciding with the first terminal of the same sensing structure 1), as represented schematically in FIG. 3, where leakage currents are designated by ILEAK.
In a known way, leakage currents may derive, for example, from one or more of the following factors: the sensing structure 1 of the MEMS microphone; the semiconductor junctions of the transistor devices that provide the switch element 12; the electrical connection between the sensing structure 1 and the corresponding read stage 11 (given that the ASIC may be provided in a distinct die or even in a distinct package); electrostatic-discharge (ESD) protection circuits that may be present in the ASIC; or other known factors (not listed here).
In any case, it is known that leakage currents are intrinsically present and may not be avoided.
The drawback associated with leakage currents (as shown in FIG. 4) is due to the voltage drop ΔV that they cause across the insulating circuit element 10, which is high in value, even in the region of some hundreds of millivolts on account of the value of resistance of the insulating circuit element 10.
Consequently, upon opening of the switch element 12 (after a time interval designated by tshort starting from the start of the start-up phase, of which FIG. 4 shows only a final portion, subsequent to a period of settling of the voltage VMEMS to the value VCP), the capacitor of the MEMS microphone has to discharge from the initial voltage value, forced by the switch element 12, equal to the voltage VCP, down to a new value, equal to VCP−ΔV, of even some hundreds of millivolts lower.
The above discharge is once again carried out with a high time constant, causing a considerable delay of time, designated by td, which determines an undesirable lengthening of the start-up time interval, designated by tstart-up.
Such long delay times may not be accepted in a wide range of situations of use of the MEMS microphone, when it is desirable to guarantee the nominal performance (and in particular a substantially constant sensitivity) with extremely short delays, both upon turning-on of the electronic device incorporating the MEMS microphone and upon re-entry from a standby or power-down condition.
As a possible solution to this drawback, the use of an insulating circuit element 10 with lower impedance, for example in the region of some tens of giga-ohms, has been proposed, thereby generating a lower voltage drop ΔV and a consequently shorter delay of time td.
However, this solution also entails an undesirable increase in noise in so far as the lower value of impedance of the insulating circuit element 10 degrades the signal-to-noise ratio (SNR) in a way not acceptable for applications in which high performance is highly desirable.